Atomic Weights IUPAC

Pure Appl. Chem., Vol. 73, No. 4, pp. 667–683, 2001.© 2001 IUPAC

667

INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY

INORGANIC CHEMISTRY DIVISIONCOMMISSION ON ATOMIC WEIGHTS AND ISOTOPIC ABUNDANCES*

ATOMIC WEIGHTS OF THE ELEMENTS 1999

(IUPAC Technical Report)

Prepared for publication byT. B. COPLEN

U.S. Geological Survey, 431 National Center, Reston, Virginia 20192, USA

*Membership of the Commission for the period 1998–1999 was as follows:Chairman: L. Schultz (Germany); Secretary: R. D. Vocke, Jr. (USA); Titular Members: J. K. Böhlke (USA); M. Ebihara (Japan); R. D. Loss (Australia); G. Ramendik (Russia); P. D. P. Taylor (Belgium); Associate Members:T. Ding (China); A. N. Halliday (USA); H.-J. Kluge (Germany); D. J. Rokop (USA); M. Stiévenard (France); S. Yoneda (Japan); National Representatives: P. De Bièvre (Belgium); J. R. de Laeter (Australia); Y. Do (Korea);N. N. Greenwood (UK); M. Shima (Japan); Y. K. Xiao (China).

Republication or reproduction of this report or its storage and/or dissemination by electronic means is permitted without theneed for formal IUPAC permission on condition that an acknowledgment, with full reference to the source along with use of thecopyright symbol ©, the name IUPAC, and the year of publication, are prominently visible. Publication of a translation intoanother language is subject to the additional condition of prior approval from the relevant IUPAC National AdheringOrganization.

Atomic weights of the elements 1999

(IUPAC Technical Report)

Abstract: The biennial review of atomic-weight, Ar(E), determinations and othercognate data have resulted in changes for the standard atomic weights of the fol-lowing elements:

From To

nitrogen 14.006 74 ± 0.000 07 14.0067 ± 0.0002sulfur 32.066 ± 0.006 32.065 ± 0.005chlorine 35.4527 ± 0.0009 35.453 ± 0.002germanium 72.61 ± 0.02 72.64 ± 0.01xenon 131.29 ± 0.02 131.293 ± 0.006erbium 167.26 ± 0.03 167.259 ± 0.003uranium 238.0289 ± 0.0001 238.028 91 ± 0.000 03

Presented are updated tables of the standard atomic weights and their uncer-tainties estimated by combining experimental uncertainties and terrestrial vari-abilities. In addition, this report again contains an updated table of relative atom-ic-mass values and half-lives of selected radioisotopes. Changes in the evaluatedisotopic abundance values from those published in 1997 are so minor that anupdated list will not be published for the year 1999.

Many elements have a different isotopic composition in some nonterrestrialmaterials. Some recent data on parent nuclides that might affect isotopic abun-dances or atomic-weight values are included in this report for the information ofthe interested scientific community.

INTRODUCTION

The Commission on Atomic Weights and Isotopic Abundances met under the chairmanship of Prof. L. Schultz from 8–10 August 1999 during the 40th IUPAC General Assembly in Berlin, Germany.The Commission decided to publish the report “Atomic Weights of the Elements 1999” as presentedhere. The resulting current Table of Standard Atomic Weights is given in alphabetical order of the prin-cipal English names in Table 1 and in order of atomic number in Table 2. The atomic weights reportedin Tables 1 and 2 are for atoms in their electronic and nuclear ground states.

The Commission reviewed the literature over the previous two years since the last report on atom-ic weights [1] and evaluated the published data on atomic weights and isotopic compositions on an ele-ment-by-element basis. The atomic weight, Ar(E), of element E can be determined from a knowledgeof the isotopic abundances and corresponding atomic masses of the nuclides of that element.Compilations of the abundances of the isotopes were published in 1998 [2], and the atomic-mass eval-uations of 1993 [3] have been used by the Commission. The Commission periodically reviews the his-tory of the atomic weight of each element, emphasizing the relevant published scientific evidence onwhich decisions have been made [4].

The Commission wishes to emphasize the need for new precise calibrated isotopic compositionmeasurements in order to improve the accuracy of the atomic weights of a number of elements, whichare still not known to a satisfactory level of accuracy. For many elements, the limited accuracy of meas-urements, however, is overshadowed by terrestrial variability, which is combined in the tabulated uncer-tainty of the atomic weights.

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© 2001 IUPAC, Pure and Applied Chemistry 73, 667–683

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For all elements for which a change in the Ar(E) value or its uncertainty, U[Ar(E)] (in parenthe-ses, following the last significant figure to which it is attributed), is recommended, the Commission bycustom makes a statement on the reason for the change and includes a list of past recommended valuesover a period in excess of the last 100 years, which are taken from Coplen and Peiser, 1998 [5]. Valuesbefore the formation of the International Committee on Atomic Weights in 1900 come from F. W. Clarke [6].

The names and symbols for those elements with atomic numbers 110 to 118 referred to in the fol-lowing tables are systematic and based on the atomic numbers of the elements recommended for provi-sional use by the IUPAC Commission on the Nomenclature of Inorganic Chemistry [7]. These systemat-ic names and symbols will be replaced by a permanent name approved by IUPAC, once the priority of dis-covery is established and the name suggested by the discoverers is examined and reviewed. The name isderived directly from the atomic number of the element using the following numerical roots:

1 un 2 bi 3 tri 4 quad 5 pent6 hex 7 sept 8 oct 9 enn 0 nil

The roots are put together in the order of the digits that make up the atomic number and termi-nated by “ium” to spell out the name. The final “n” of “enn” is deleted when it occurs before “nil,” andthe final “i” of “bi” and of “tri” is deleted when it occurs before “ium.”

COMMENTS ON SOME ATOMIC WEIGHTS AND ANNOTATIONS

Nitrogen

The Commission has changed the recommended value for the standard atomic weight of nitrogen toAr(N) = 14.0067(2) based on an evaluation of the variation in isotopic abundance of naturally occurringnitrogen-bearing substances. The Commission decided that the uncertainty of Ar(N) should be increasedso that the implied range in relation to terrestrial variability is similar to that of other elements with sig-nificant natural isotopic variation, such as hydrogen, oxygen, and carbon. However, footnote “g”remains in Tables 1 and 2 to warn users that highly unusual naturally occurring nitrogen-bearing sub-stances can be found with an atomic weight that falls outside the implied range. The previous value,Ar(N) = 14.006 74(7), was adopted by the Commission in 1985 [4] and was based on a change in theprocedures for reporting uncertainties, taking into account mass spectrometric measurements of nitro-gen in air by Junk and Svec in 1958 [8]. Historical values of Ar(N) include [5]: 1882, 14.03; 1895,14.05; 1896, 14.04; 1907, 14.01; 1919, 14.008; 1961, 14.0067(3); and 1969, 14.0067(1).

Sulfur

The Commission has changed the recommended value for the standard atomic weight of sulfur toAr(S) = 32.065(5) based on a redetermination of the atomic weight of the Cañon Diablo troilite (CDT)reference material (32.064) and a re-evaluation of the variation in sulfur isotopic abundances of natu-rally occurring S-bearing substances. The new standard atomic weight and its uncertainty includealmost all known values in terrestrial materials; however, footnote “g” remains to alert users to the exis-tence of rare materials with atomic weights outside that range. Sulfur has a bimodal natural atomic-weight distribution so that in practice one is more likely to find materials with either lower or higheratomic-weight values than 32.065. New calibrated mass spectrometric measurements of the isotopiccomposition of the IAEA-S-1 silver sulfide reference material [9,10] indicate that the atomic weight ofCDT is 32.064, which is in agreement with the atomic weight of meteoritic sulfur determined byMcNamara and Thode [11]. The previous standard atomic weight value, Ar(S) = 32.066(6), was adopt-ed by the Commission in 1983 based on a compilation of isotopic variations of naturally occurringmaterials. Historical values of Ar(S) include [5]: 1882, 32.06; 1896, 32.07; 1903, 32.06; 1909, 32.07;1916, 32.06; 1925, 32.064; 1931, 32.06; 1947, 32.056; 1961, 32.064(3); and 1969, 32.06(1).


Atomic weights of the elements 1999 669

Table 1Standard atomic weights 1999.

[Scaled to Ar(12C) = 12, where 12C is a neutral atom in its nuclear and electronic ground state.]

The atomic weights of many elements are not invariant but depend on the origin and treatment of thematerial. The standard values of Ar(E) and the uncertainties (in parentheses, following the last significantfigure to which they are attributed) apply to elements of natural terrestrial origin. The footnotes to thistable elaborate the types of variation which may occur for individual elements and which may be largerthan the listed uncertainties of values of Ar(E). Names of elements with atomic number 110 to 118 areprovisional.

Alphabetical order in English

Atomic AtomicName Symbol Number Weight Footnotes

Actinium* Ac 89Aluminium (Aluminum) Al 13 26.981 538(2)Americium* Am 95Antimony (Stibium) Sb 51 121.760(1) gArgon Ar 18 39.948(1) g rArsenic As 33 74.921 60(2)Astatine* At 85Barium Ba 56 137.327(7)Berkelium* Bk 97Beryllium Be 4 9.012 182(3)Bismuth Bi 83 208.980 38(2)Bohrium* Bh 107Boron B 5 10.811(7) g m rBromine Br 35 79.904(1)Cadmium Cd 48 112.411(8) gCaesium (Cesium) Cs 55 132.905 45(2)Calcium Ca 20 40.078(4) gCalifornium* Cf 98Carbon C 6 12.0107(8) g rCerium Ce 58 140.116(1) gChlorine Cl 17 35.453(2) g m rChromium Cr 24 51.9961(6)Cobalt Co 27 58.933 200(9)Copper (Cuprum) Cu 29 63.546(3) rCurium* Cm 96Dubnium* Db 105Dysprosium Dy 66 162.50(3) gEinsteinium* Es 99Erbium Er 68 167.259(3) gEuropium Eu 63 151.964(1) gFermium* Fm 100Fluorine F 9 18.998 4032(5)Francium* Fr 87Gadolinium Gd 64 157.25(3) gGallium Ga 31 69.723(1)Germanium Ge 32 72.64(1)Gold (Aurum) Au 79 196.966 55(2)Hafnium Hf 72 178.49(2)Hassium* Hs 108

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Table 1 (Continued)


Helium He 2 4.002 602(2) g rHolmium Ho 67 164.930 32(2)Hydrogen H 1 1.007 94(7) g m rIndium In 49 114.818(3)Iodine I 53 126.904 47(3)Iridium Ir 77 192.217(3)Iron (Ferrum) Fe 26 55.845(2)Krypton Kr 36 83.80(1) g mLanthanum La 57 138.9055(2) gLawrencium* Lr 103Lead (Plumbum) Pb 82 207.2(1) g rLithium Li 3 [6.941(2)]† g m rLutetium Lu 71 174.967(1) gMagnesium Mg 12 24.3050(6)Manganese Mn 25 54.938 049(9)Meitnerium* Mt 109Mendelevium* Md 101Mercury (Hydrargyrum) Hg 80 200.59(2)Molybdenum Mo 42 95.94(1) gNeodymium Nd 60 144.24(3) gNeon Ne 10 20.1797(6) g mNeptunium* Np 93Nickel Ni 28 58.6934(2)Niobium Nb 41 92.906 38(2)Nitrogen N 7 14.0067(2) g rNobelium* No 102Osmium Os 76 190.23(3) gOxygen O 8 15.9994(3) g rPalladium Pd 46 106.42(1) gPhosphorus P 15 30.973 761(2)Platinum Pt 78 195.078(2)Plutonium* Pu 94Polonium* Po 84Potassium (Kalium) K 19 39.0983(1)Praseodymium Pr 59 140.907 65(2)Promethium* Pm 61Protactinium* Pa 91 231.035 88(2)Radium* Ra 88Radon* Rn 86Rhenium Re 75 186.207(1)Rhodium Rh 45 102.905 50(2)Rubidium Rb 37 85.4678(3) gRuthenium Ru 44 101.07(2) gRutherfordium* Rf 104Samarium Sm 62 150.36(3) gScandium Sc 21 44.955 910(8)Seaborgium* Sg 106Selenium Se 34 78.96(3) r



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Table 1 (Continued)


Silicon Si 14 28.0855(3) rSilver (Argentum) Ag 47 107.8682(2) gSodium (Natrium) Na 11 22.989 770(2)Strontium Sr 38 87.62(1) g rSulfur S 16 32.065(5) g rTantalum Ta 73 180.9479(1)Technetium* Tc 43Tellurium Te 52 127.60(3) gTerbium Tb 65 158.925 34(2)Thallium Tl 81 204.3833(2)Thorium* Th 90 232.0381(1) gThulium Tm 69 168.934 21(2)Tin (Stannum) Sn 50 118.710(7) gTitanium Ti 22 47.867(1)Tungsten (Wolfram) W 74 183.84(1)Ununbium* Uub 112Ununhexium* Uuh 116Ununnilium* Uun 110Ununoctium* Uuo 118Ununquadium* Uuq 114Unununium* Uuu 111Uranium* U 92 238.028 91(3) g mVanadium V 23 50.9415(1)Xenon Xe 54 131.293(6) g mYtterbium Yb 70 173.04(3) gYttrium Y 39 88.905 85(2)Zinc Zn 30 65.39(2)Zirconium Zr 40 91.224(2) g

*Element has no stable nuclides. One or more well-known isotopes are given in Table 3 with the appropriate relativeatomic mass and half-life. However, three such elements (Th, Pa, and U) do have a characteristic terrestrial isotopiccomposition, and for these an atomic weight is tabulated.†Commercially available Li materials have atomic weights that range between 6.939 and 6.996; if a more accuratevalue is required, it must be determined for the specific material.g geological specimens are known in which the element has an isotopic composition outside the limits for nor-

mal material. The difference between the atomic weight of the element in such specimens and that given in thetable may exceed the stated uncertainty.

m modified isotopic compositions may be found in commercially available material because it has been subject-ed to an undisclosed or inadvertent isotopic fractionation. Substantial deviations in atomic weight of the ele-ment from that given in the table can occur.

r range in isotopic composition of normal terrestrial material prevents a more precise Ar(E) being given; the tab-ulated Ar(E) value should be applicable to any normal material.

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Table 2Standard atomic weights 1999.

[Scaled to Ar(12C) = 12, where 12C is a neutral atom in its nuclear and electronic ground state]

The atomic weights of many elements are not invariant but depend on the origin and treatment of thematerial. The standard values of Ar(E) and the uncertainties (in parentheses, following the last signifi-cant figure to which they are attributed) apply to elements of natural terrestrial origin. The footnotes tothis table elaborate the types of variation which may occur for individual elements and which may belarger than the listed uncertainties of values of Ar(E). Names of elements with atomic number 110 to118 are provisional.

Order of atomic number

Atomic AtomicNumber Name Symbol Weight Footnotes

1 Hydrogen H 1.007 94(7) g m r2 Helium He 4.002 602(2) g r3 Lithium Li [6.941(2)]† g m r4 Beryllium Be 9.012 182(3)5 Boron B 10.811(7) g m r6 Carbon C 12.0107(8) g r7 Nitrogen N 14.0067(2) g r8 Oxygen O 15.9994(3) g r9 Fluorine F 18.998 4032(5)

10 Neon Ne 20.1797(6) g m11 Sodium (Natrium) Na 22.989 770(2)12 Magnesium Mg 24.3050(6)13 Aluminium (Aluminum) Al 26.981 538(2)14 Silicon Si 28.0855(3) r15 Phosphorus P 30.973 761(2)16 Sulfur S 32.065(5) g r17 Chlorine Cl 35.453(2) g m r18 Argon Ar 39.948(1) g r19 Potassium (Kalium) K 39.0983(1)20 Calcium Ca 40.078(4) g21 Scandium Sc 44.955 910(8)22 Titanium Ti 47.867(1)23 Vanadium V 50.9415(1)24 Chromium Cr 51.9961(6)25 Manganese Mn 54.938 049(9)26 Iron (Ferrum) Fe 55.845(2)27 Cobalt Co 58.933 200(9)28 Nickel Ni 58.6934(2)29 Copper (Cuprum) Cu 63.546(3) r30 Zinc Zn 65.39(2)31 Gallium Ga 69.723(1)32 Germanium Ge 72.64(1)33 Arsenic As 74.921 60(2)34 Selenium Se 78.96(3) r35 Bromine Br 79.904(1)36 Krypton Kr 83.80(1) g m37 Rubidium Rb 85.4678(3) g38 Strontium Sr 87.62(1) g r



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Table 2 (Continued)


39 Yttrium Y 88.905 85(2)40 Zirconium Zr 91.224(2) g41 Niobium Nb 92.906 38(2)42 Molybdenum Mo 95.94(1) g43 Technetium* Tc44 Ruthenium Ru 101.07(2) g45 Rhodium Rh 102.905 50(2)46 Palladium Pd 106.42(1) g47 Silver (Argentum) Ag 107.8682(2) g48 Cadmium Cd 112.411(8) g49 Indium In 114.818(3)50 Tin (Stannum) Sn 118.710(7) g51 Antimony (Stibium) Sb 121.760(1) g52 Tellurium Te 127.60(3) g53 Iodine I 126.904 47(3)54 Xenon Xe 131.293(6) g m55 Caesium (Cesium) Cs 132.905 45(2)56 Barium Ba 137.327(7)57 Lanthanum La 138.9055(2) g58 Cerium Ce 140.116(1) g59 Praseodymium Pr 140.907 65(2)60 Neodymium Nd 144.24(3) g61 Promethium* Pm62 Samarium Sm 150.36(3) g63 Europium Eu 151.964(1) g64 Gadolinium Gd 157.25(3) g65 Terbium Tb 158.925 34(2)66 Dysprosium Dy 162.50(3) g67 Holmium Ho 164.930 32(2)68 Erbium Er 167.259(3) g69 Thulium Tm 168.934 21(2)70 Ytterbium Yb 173.04(3) g71 Lutetium Lu 174.967(1) g72 Hafnium Hf 178.49(2)73 Tantalum Ta 180.9479(1)74 Tungsten (Wolfram) W 183.84(1)75 Rhenium Re 186.207(1)76 Osmium Os 190.23(3) g77 Iridium Ir 192.217(3)78 Platinum Pt 195.078(2)79 Gold (Aurum) Au 196.966 55(2)80 Mercury (Hydrargyrum) Hg 200.59(2)81 Thallium Tl 204.3833(2)82 Lead (Plumbum) Pb 207.2(1) g r83 Bismuth Bi 208.980 38(2)84 Polonium* Po85 Astatine* At86 Radon* Rn87 Francium* Fr

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Table 2 (Continued)


88 Radium* Ra89 Actinium* Ac90 Thorium* Th 232.0381(1) g91 Protactinium* Pa 231.035 88(2)92 Uranium* U 238.028 91(3) g m93 Neptunium* Np94 Plutonium* Pu95 Americium* Am96 Curium* Cm97 Berkelium* Bk98 Californium* Cf99 Einsteinium* Es100 Fermium* Fm101 Mendelevium* Md102 Nobelium* No103 Lawrencium* Lr104 Rutherfordium* Rf105 Dubnium* Db106 Seaborgium* Sg107 Bohrium* Bh108 Hassium* Hs109 Meitnerium* Mt110 Ununnilium* Uun111 Unununium* Uuu112 Ununbium* Uub114 Ununquadium* Uuq116 Ununhexium* Uuh118 Ununoctium* Uuo

*Element has no stable nuclides. One or more well-known isotopes are given in Table 3 with the appropriate relativeatomic mass and half-life. However, three such elements (Th, Pa, and U) do have a characteristic terrestrial isotopiccomposition, and for these an atomic weight is tabulated.†Commercially available Li materials have atomic weights that range between 6.939 and 6.996; if a more accuratevalue is required, it must be determined for the specific material.g geological specimens are known in which the element has an isotopic composition outside the limits

for normal material. The difference between the atomic weight of the element in such specimens andthat given in the Table may exceed the stated uncertainty.

m modified isotopic compositions may be found in commercially available material because it has beensubjected to an undisclosed or inadvertent isotopic fractionation. Substantial deviations in atomicweight of the element from that given in the Table can occur.

r range in isotopic composition of normal terrestrial material prevents a more precise Ar(E) being given;the tabulated Ar(E) value should be applicable to any normal material.

Chlorine

The Commission has changed the recommended value for the standard atomic weight of chlorine toAr(Cl) = 35.453(2), based on an evaluation of the variation in isotopic abundances of naturally occur-ring chlorine-bearing substances. The Commission decided that the uncertainty of Ar(Cl) should beincreased so that the implied range in relation to terrestrial variability is similar to that of other elementswith significant natural isotopic variation, such as hydrogen, oxygen, and carbon. To reflect the fact that



the new atomic weight of Cl and its uncertainty were assigned on the basis of known variability, thefootnote “r” was added to Tables 1 and 2. The footnote “g” has been added to Tables 1 and 2 to alertusers that highly unusual naturally occurring chlorine-bearing substances can be found with an atomicweight that falls outside the implied range. The previous value, Ar(Cl) = 35.4527(9), was adopted bythe Commission in 1985 [4] and was based on a change in the procedures for reporting uncertainties,taking into account the calibrated mass spectrometric measurements of chlorine by Shields et al. [12].Historical values of Ar(Cl) include [5]: 1882, 35.45; 1909, 35.46; 1925, 35.457; 1961, 35.453; and 1969,35.453(1).

Germanium

The Commission has changed the recommended value for the standard atomic weight of germanium toAr(Ge) = 72.64(1), based on calibrated mass spectrometric measurements by Chang et al. [13].Measurements were carried out on five germanium samples of high purity (Ge metal and GeO2) fromAmerica, Europe, and Asia using positive thermal ionization mass spectrometry. The previous value ofAr(Ge) = 72.61(2) was assigned by the Commission in 1985 [4], which considered the discrepancybetween atomic weights determined by mass spectrometry and chemical methods [4,14]. By recent highquality mass spectrometric measurements, the mass spectrometric data are confirmed. Historical valuesof Ar(Ge) include [5]: 1894, 72.3; 1897, 72.48; 1900, 72.5; 1925, 72.60; 1961, 72.59; and 1969,72.59(3).

Xenon

The Commission has changed the recommended value for the standard atomic weight of xenon toAr(Xe) = 131.293(6), based on a new calibrated mass spectrometric measurement of Valkiers et al. [15]on a tank of purified xenon. The footnote “g” in Tables 1 and 2 arises from the presence of naturallyoccurring fission products found at fossil reactors at The Gabon, Africa. The previous value, Ar(Xe) =131.29(2), was adopted by the Commission in 1985 [4] and was based on a change in the proceduresfor reporting uncertainties, and it took into account the mass spectrometric measurements of Nier [16].Historical values of Ar(Xe) include [5]: 1902, 128; 1910, 130.7; 1911, 130.2; 1932, 131.3; 1955,131.30; 1969, 131.30(1); and 1979, 131.29(3).

Erbium

The Commission has changed the recommended value for the standard atomic weight of erbium toAr(Er) = 167.259(3), based on calibrated measurements with highly enriched erbium isotopes using pos-itive thermal ionization mass spectrometry and a multi-collector system [17]. The value for the uncer-tainty takes into account the lack of adequate quantification of collector calibration. Erbium isolatedfrom a kaolinite mineral and from four different commercially available Er-bearing materials did notshow any isotope variation. Nevertheless, footnote “g” in Tables 1 and 2 arises from the presence of nat-urally occurring fission products found at fossil reactors at The Gabon, Africa. The previous value,Ar(Er) = 167.26(3), was adopted by the Commission in 1961, based on isotopic abundance measure-ments of erbium by Hayden et al. [18] and Leland [19] and atomic masses of Bhanot et al. [20].Historical values of Ar(Er) include [5]: 1882, 166.27; 1894, 166.3; 1897, 166.32; 1900, 166.0; 1909,167.4; 1912, 167.7; 1931, 167.64; 1934, 165.20; 1935, 167.84; 1938, 167.2; 1955, 167.22; 1961,167.26; 1969, 167.26(3).

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Uranium

The Commission has changed the recommended value for the standard atomic weight of uranium toAr(U) = 238.02891(3) based on the calibrated mass spectrometric determinations by Richter et al. [21].These measurements reflect the improved capability of current double focusing mass spectrometerswith pulse counting ion-detection systems for determination of low abundance isotopes and updated gasmass spectrometry measurements of the major isotopes. All measurements were based on internal nor-malization against well characterized isotope standards. A set of six ore samples collected worldwidewas utilized for the measurements. That value applies to uranium as found in normal terrestrial sources,except as discovered in one locality in Africa (The Gabon at Oklo), hence the footnote “g” in Tables 1and 2. The previous value, Ar(U) = 238.0289(1), was adopted by the Commission in 1979 taking intoaccount the studies on isotopic variation by Cowan and Adler [22] and Smith and Jackson [23], andmass spectrometric measurements of White et al. [24] and Greene et al. [25]. Historical values ofAr(U) include [5]: 1882, 239.03; 1894, 239.6; 1896, 239.59; 1900, 239.6; 1903, 238.5; 1916, 238.2;1925, 238.17; 1931, 238.14; 1937, 238.07; 1961, 238.03; and 1969, 238.029(1).

RELATIVE ATOMIC-MASS VALUES AND HALF-LIVES OF SELECTED RADIONUCLIDES

For elements that have no stable or long-lived nuclides, the data on radioactive half-lives and relativeatomic-mass values for the nuclides of interest and importance have been evaluated, and the recom-mended values and uncertainties are listed in Table 3.

As has been the custom in the past, the Commission publishes a table of relative atomic-massvalues and half-lives of selected radionuclides. The Commission has no prime responsibility for thedissemination of such values. There is no general agreement on which of the nuclides of the radioac-tive elements is, or is likely to be judged, “important”. Various criteria such as “longest half-life”,“production in quantity”, “used commercially”, have been applied in the past to the Commission’schoice.

The information contained in this table will enable the user to calculate the atomic weight forradioactive materials with a variety of isotopic compositions. Atomic-mass values have been takenfrom the 1997 Atomic Mass Table [3]. Some of these half-lives have already been documented[26–29].

NONTERRESTRIAL DATA

The isotopic abundance of elements in many nonterrestrial samples within the solar system can bemeasured directly by analysis of meteorites and other interplanetary materials. In recent years, theincreasing sophistication of analytical instrumentation and techniques have enabled the isotopic com-position of submicron sized components in nonterrestrial samples to be determined. At the same time,there has also been an increase in the use of on-board spacecraft and ground-based astronomical spec-trometers to measure isotopic abundances remotely. These advances have substantially increased thenumber of isotopic composition data for nonterrestrial materials.

The extensive analysis of nonterrestrial materials has continued to show that many elements innonterrestrial materials have different isotopic compositions from those in terrestrial samples.Although most of the reported differences in the isotopic compositions in nonterrestrial materials aresmall compared with corresponding differences in normal terrestrial materials, some variations arequite large. For this reason, scientists who deal with the chemical analysis of nonterrestrial samplesshould exercise caution when the isotopic composition or the atomic weight of nonterrestrial samplesis needed.



Table 3Relative atomic masses and half-lives of selected radionuclides.

[Prepared, as in previous years, by N. E. Holden, a former Commission member; a = year; d = day; h = hour;min = minute; s = second. Names of elements with atomic number 110 to 118 are provisional.]

Atomic Element Element Mass Atomic Half-life UnitNumber Name Symbol No. Mass

43 Technetium Tc 97 96.9064 4.0(3) × 106 a98 97.9072 6.6(10) × 106 a99 98.9063 2.1(3) × 105 a

61 Promethium Pm 145 144.9127 17.7(4) a147 146.9151 2.623(3) a

84 Polonium Po 209 208.9824 102(5) a210 209.9829 138.4(1) d

85 Astatine At 210 209.9871 8.1(4) h211 210.9875 7.21(1) h

86 Radon Rn 211 210.9906 14.6(2) h220 220.0114 55.6(1) s222 222.0176 3.823(4) d

87 Francium Fr 223 223.0197 22.0(1) min88 Radium Ra 223 223.0185 11.43(1) d

224 224.0202 3.66(2) d226 226.0254 1599(4) a228 228.0311 5.75(3) a

89 Actinium Ac 227 227.0277 21.77(2) a90 Thorium Th 230 230.0331 7.54(3) × 104 a

232 232.0381 1.40(1) × 1010 a91 Protactinium Pa 231 231.0359 3.25(1) × 104 a92 Uranium U 233 233.0396 1.592(2) × 105 a

234 234.0409 2.455(6) × 105 a235 235.0439 7.04(1) × 108 a236 236.0456 2.342(4) × 107 a238 238.0508 4.468(3) × 109 a

93 Neptunium Np 237 237.0482 2.14(1) × 106 a239 239.0529 2.355(6) d

94 Plutonium Pu 238 238.0496 87.7(1) a239 239.0522 2.410(3) × 104 a240 240.0538 6.56(1) × 103 a241 241.0568 14.4(1) a242 242.0587 3.75(2) × 105 a244 244.0642 8.00(9) × 107 a

95 Americium Am 241 241.0568 432.7(6) a243 243.0614 7.37(2) × 103 a

96 Curium Cm 243 243.0614 29.1(1) a244 244.0627 18.1(1) a245 245.0655 8.48(6) × 103 a246 246.0672 4.76(4) × 103 a247 247.0704 1.56(5) × 107 a248 248.0723 3.48(6) × 105 a

97 Berkelium Bk 247 247.0703 1.4(3) × 103 a249 249.0750 3.26(3) × 102 d

98 Californium Cf 249 249.0749 351(2) a250 250.0764 13.1(1) a251 251.0796 9.0(5) × 102 a

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Table 3 (Continued)

Atomic Element Element Mass Atomic Half-life UnitNumber Name Symbol No. Mass

252 252.0816 2.64(1) a99 Einsteinium Es 252 252.0830 472(2) d100 Fermium Fm 257 257.0951 100.5(2) d101 Mendelevium Md 258 258.0984 51.5(3) d

260 260.1037 27.8(3) d102 Nobelium No 259 259.1010 58(5) min103 Lawrencium Lr 262 262.1097 3.6(3) h104 Rutherfordium Rf 261 261.1088 1.3a min105 Dubnium Db 262 262.1141 34(5) s106 Seaborgium Sg 266 266.1219 ~21a s107 Bohrium Bh 264 264.12 0.44a s108 Hassium Hs 277 16.5a,b min109 Meitnerium Mt 268 268.1388 0.070a,b s110 Ununnilium Uun 281 1.6a,b min111 Unununium Uuu 272 272.1535 1.5a,b× 10-3 s112 Ununbium Uub 285 15.4a,b min114 Ununquadium Uuq 289 30.4a,b s116 Ununhexium Uuh 289 0.60a,b× 10-3 s118 Ununoctium Uuo 293 0.12a,b× 10-3 s

a The uncertainties of these elements are unsymmetric.b The value given is determined from only a few decays.

Most isotopic variations observed in nonterrestrial materials are currently explained by the fol-lowing processes:

(i) Mass fractionation. Mass-dependent isotopic fractionation can also be observed in many terres-trial materials, but the magnitude of mass-dependent isotopic fractionation in nonterrestrial sam-ples is generally larger than that found in terrestrial samples. Mass-dependent isotopic fractiona-tion observed in nonterrestrial samples is mostly due to volatilization or condensation, whichmost likely occurred at the early stage of the formation of the solar system. Mass-dependent iso-topic fractionation may also have occurred at later stages in the evolution of the solar system bychemical processes such as the formation of organic compounds.

(ii) Nuclear reaction. Nuclear reactions triggered by cosmic rays can alter the isotopic compositionof not only nonterrestrial but also terrestrial materials. This effect in terrestrial samples is nor-mally negligible, mainly because of effective shielding against cosmic rays by the Earth’s mag-netosphere and atmosphere. In contrast, nonterrestrial materials are often exposed to sufficientsolar and galactic cosmic ray fluxes to cause significant nuclear spallation reactions, which in turntrigger secondary low-energy neutron capture reactions at specific depths from the surface or nearthe surface of samples.

(iii) Radioactive decay. Isotopic anomalies due to radioactive decay are not limited to nonterrestrialsamples. Nevertheless, their effect can be clearly observed in nonterrestrial samples both in thedegree of the alteration of isotopic abundances and in the variety of radioactive nuclides.Enrichment in decay products is the most common effect confirmed in the nonterrestrial samples.The effect is caused by long-lived nuclides (primary radioactive nuclides), whose half-lives arecomparable to the age of our solar system (4.56 × 109 a). These nuclides are commonly used ingeochronology and cosmochronology.



The next observed effect caused by radioactive decay is due to parent nuclides that have decayed.While such nuclides are no longer present in the solar system, their former presence in nonter-restrial materials can be demonstrated by excesses in decay products, which together with theirparent(s) have been part of a closed isotopic system. Their measurement provides valuable infor-mation related to the time from their final nucleosynthetic contribution to the solar system to theirincorporation to solar system materials (also known as the solar system formation interval).In addition to these two major effects, some minor effects can be observed in nonterrestrial sam-ples such as those caused by double ß-decay of long-lived radionuclides and nuclear fission(spontaneous and neutron-induced).

(iv) Nucleosynthesis. The most significant isotopic alteration effect observed in nonterrestrial materi-als (mainly meteorites) is due to nucleosynthesis. Prior to the availability of secondary ion massspectrometry (SIMS), the majority of isotopic effects due to nucleosynthesis were known to bepresent in only a few component samples present in a limited number of rare meteorites. Themajority of these isotopic abundance variations were best ascribed to explosive nucleosynthesis

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Fig. 1 Mass spectrum of Xe extracted from the Richardton meteorite [31]. Horizontal lines indicate the comparisonspectrum of terrestrial Xe normalized to 132Xe. A large excess at 129Xe is due to radioactive decay of a parentnuclide, 129I (half-life = 1.6 × 107 a).

effects. Some light elements, such as oxygen and noble gas elements, showed ubiquitous isotopicanomalies (which were also considered to be related to nucleosynthetic effects) in virtually allkinds of meteoritic materials. The extensive application of the SIMS technique now shows that alarge number of extra-solar materials appear to contain significant isotope anomalies. This indi-cates that several extra-solar materials appear to have survived the formation of the solar systemeither within meteorites or their components. Some of these components demonstrate huge iso-topic variations, which are believed to be due to a wide range of nucleosynthetic effects occurringat various stages of stellar evolution. A number of examples have been listed in the previous Pureand Applied Chemistry(PAC) publications [30].

Figure 1 shows an example of how the radioactive decay of a parent nuclide, 129I in this case,alters the isotopic composition of nonterrestrial materials. 129I is a typical parent nuclide and decays to129Xe with a half-life of 1.6 × 107 a. The presence of 129I at the early solar system was first confirmedin 1960 by Reynolds, who observed an excess of 129Xe in noble gas extracted from the Richardtonmeteorite [31]. Since then, more than ten long-living radionuclides with half-lives less than the age ofEarth have been proposed to alter the isotopic abundance of their daughter nuclide after their decay(Table 4). The isotopic composition of Xe tends to suffer other alteration processes, such as nuclearreactions (cosmic-ray-produced spallation reaction and nuclear fission reaction) and mass-dependentisotopic fractionation. Meteoritic Xe thus occasionally shows a highly complex isotopic composition.

Table 4Examples of measurements of anomalous isotopic compositions in extraterrestrial materialsarising from the earlier decay of radioisotopes.

Parent Half-life Decay Mode Daughter Absolute Difference RefsNuclide in 106 a Nuclide Between the Observed

Isotopic Ratio and ThatFound in Normal

Terrestrial Materials

26Al 0.72 ß+, EC 26Mg 0.095 (26Mg/24Mg) 3236Cl 0.301 ß- 36Ar 0.066 (38Ar/36Ar) 3341Ca 0.10 EC 41K 5.7 (41K/39K) 3453Mn 3.7 EC 53Cr 0.0018 (53Cr/52Cr) 3560Fe ~0.1 2ß- 60Ni 0.005 (60Ni/58Ni) 36107Pd 6.5 ß- 107Ag 0.04 (107Ag/109Ag) 37129I 16 ß- 129Xe 0.48 (129Xe/131Xe) 31135Cs 3 ß- 135Ba -0.0002 (135Ba/138Ba) 38146Sm 103 α 142Nd 0.000 06 (142Nd/144Nd) 39182Hf 9 s, r 182W -0.0003 (182W/184W) 40244Pu 82 SF 131–136Xe 0.369 (136Xe/130Xe) 41247Cm 15.8 α, ß- 235U <0.0074 (235U/238U) 42

The Commission does not attempt to systematically review the literature on the isotopic compo-sitions of nonterrestrial materials in this report. Those who are interested in more comprehensivereviews, including specific data and additional references, should refer to Shima [43] and Shima andEbihara [44]. A more detailed report of isotopic measurement published in this field during the lastdecade is in preparation.



OTHER PROJECTS OF THE COMMISSION

At intervals of about six to eight years, the Commission’s Subcommittee for Isotopic AbundanceMeasurements publishes a summary of its biennial review of isotopic compositions of the elements asdetermined by mass spectrometry and other relevant methods. The subcommittee distributed their sum-mary report at Berlin, titled “Isotopic compositions of the elements 1997” [2]. The very few and minor1999 changes of the evaluated best values for some isotopic abundances do not justify the publicationof an updated table, which, however, is always currently maintained by and available on request fromthe Commission.

The rules that the Commission employs in assigning atomic-weight values are found in theCommission’s Technical Booklet. J. de Laeter incorporated decisions made at the previous GeneralAssembly at Geneva to produce the 5th edition of the Commission’s Technical Booklet, which he dis-tributed. A new addition to this booklet is a paper on the reliability of the Avogadro constant, the fac-tor that relates measurements expressed in two SI base units (mass and amount of substance) by De Bièvre and Peiser [45].

The Subcommittee for Natural Isotopic Fractionation presented a report that was discussed andmodified during the Subcommittee’s meeting in Berlin, Germany, prior to the IUPAC GeneralAssembly in Berlin. The Subcommittee will submit a report to PAC, consisting primarily of plots(where possible) that show the variation in natural isotopic abundance and atomic weight for the ele-ments H, Li, B, C, N, O, Si, S, Cl, Fe, Cu, Se, Pd, and Te. A companion report that is expected to belonger will include numerous references and will be published as a U.S. Geological Survey Open-FileReport.

J. de Laeter reported on plans and progress of the Commission-approved update to the 1984Element By Element Review [4]. Publication of this important document is planned before the nextIUPAC General Assembly in 2001 in Brisbane.

Recognizing that many of the terms the Commission uses are missing from the second edition ofIUPAC’s book on nomenclature (the “Gold Book” [46]), the Commission asked G. Ramendik to pre-pare a list of such terms and obtain review for submission to IUPAC nomenclature commissions andinclusion in the 3rd edition of the Gold Book.

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